In order to vendor-in libnetwork to docker, we need to remove the swarm
dependency even though it is used as library. using this PR, a new build
flag libnetwork_discovery is introduced in order to avoid pulling in the
unused hostdiscovery functionality into docker.
We are working with the Swarm project to see if we can modularize the
discovery package to become independent so that we can include them as a
vendor-in package in docker.
Signed-off-by: Madhu Venugopal <madhu@docker.com>